You can mount the double gearbox to our round robot chassis without any modifications if you use the two gear ratios that position the axle closer to the motors (344.2:1 and 38.2:1). Using the the two gear ratios that position the axle farther from the motors (114.7:1 and 12.7:1) will cause the wheels to hit the chassis. Please note that the axle of the double gearbox will not go through the center of the circular chassis; it will be slightly off-center. The chassis was designed for the twin-motor gearbox, which looks a lot like the double gearbox but does not have the same dimensions (we designed the chassis before the double gearbox existed); if you use the twin-motor gearbox, the axle will go through the center of the chassis.